fix(shard-distributor): Handle ErrShardAlreadyAssigned as success in assignEphemeralShard

[jakobht]

What changed?

Handle ErrShardAlreadyAssigned as a successful response in assignEphemeralShard instead of returning an InternalServiceError. Added Metadata field to ErrShardAlreadyAssigned to include executor metadata directly in the error.

Why?

When multiple concurrent requests try to assign the same ephemeral shard, the first succeeds and subsequent ones receive ErrShardAlreadyAssigned. Previously this bubbled up as an InternalServiceError, causing unnecessary errors for clients. Since shard assignment is idempotent, we should return the already-assigned owner as a successful response. The error now includes metadata to avoid an extra GetExecutor DB call.

How did you test it?

go test -v ./service/sharddistributor/handler/... -run TestGetShardOwner
go test -v ./service/sharddistributor/store/...

Potential risks

N/A - This is a purely additive change that treats an existing error condition as success.

Release notes

N/A - Internal implementation detail.

Documentation Changes

N/A

[gitar-bot]

Code Review ✅ Approved

Clean, well-scoped fix that correctly treats ErrShardAlreadyAssigned as a successful idempotent response. The error-handling flow, metadata propagation, and test coverage are all solid.

Rules ✅ All requirements met

Repository Rules

✅ PR Description Quality Standards: PR description includes Intent & Rationale, Implementation Analysis, Codebase Context, Risk Assessment, and clear testing details

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